1. Field of the Invention:
The present invention is directed generally to health care lighting fixtures and, more specifically, to a dual resonant frequency power supply for a surgical room arc lamp.
2. Description of the Prior Art:
Apparatus exist for starting and operating arc lamps used in a variety of applications. Because of the significantly different operating conditions exhibited by arc lamps between the start condition and the full run condition, present power supply apparatus for arc lamps incorporate a concept which involves combining separate devices for starting the lamp and operating the lamp in the full run condition. This concept involves providing a "starting" voltage signal of one magnitude and frequency for a predetermined period of time and then switching to a "running" voltage signal of another magnitude and frequency.
The concept used in these present apparatus exhibits significant drawbacks. The practice of combining two devices is excessively complex and does not provide for a smooth crossover from one operating condition to the other. In addition, the practice of providing "starting" condition voltages for a predetermined period of time and then switching to "running" condition voltages suffers from the drawback that the lamp is subject to the higher voltages for "starting" longer than is normally necessary. The result is that the life of the lamp is greatly reduced.
Accordingly, the need exists for a power supply device which can operate an arc lamp in both the "starting" and "running" conditions in a dependable and efficient manner. The apparatus of the present invention provides continuous current to the arc lamp and, therefore, does not require restart on each half-cycle as the alternating voltage passes through zero volts. The dual resonance frequency circuit of the present invention also provides a near-unity power factor during both lamp "starting" and "running" conditions resulting in minimum circuit power losses and the ability to use passive cooling means, smaller lower-cost power transformers and smaller packaging size. The automated frequency switchover from the "starting" to "running" condition eliminates arc lamp electrode degradation during virgin lamp starting or subsequent lamp starting by providing the appropriate power during the "starting" to "running" phase transitions. Furthermore, the apparatus of the present invention provides pure sinusoidal voltages at ultrasonic frequencies which nearly eliminates EMI, a practical requirement in health care applications.